Until recently, braking systems employed in connection with towed vehicles such as horse trailers, for example, were usually one of three conventional types of systems. These include: electric braking systems, surge hydraulic braking systems, and vacuum over hydraulic braking systems. The electric type braking system conventionally includes electromagnets mounted inside the brake drum of each wheel of the towed vehicle, which upon application of electric current attach themselves to an armature plate mounted inside the brake drum. The rotating motion of the wheel then carries the attached electromagnet with it causing an armature attached to the electromagent to rotate and expand braking bands against the brake drum. Electric current to the electromagnet is supplied via a rheostat activated by hydraulic pressure from the brakes of the towing vehicle or by a hand control mounted in the passenger compartment of the towing vehicle. The disadvantages of such a system include the fact that during normal operation braking pressure is supplied first to the hydraulic brakes of the towing vehicle and then, as the electric brakes of the towed vehicle are activated, a noticeable lurch occurs which may affect the control of the operator of the towing vehicle during braking. Further, variance in electromagnetic strength among the electromagnets in each wheel may cause uneven braking and further control problems. Such a system is also prone to electrical wiring problems because electrical wires carrying current to each electromagnet are exposed between the wheel and the frame of the towed vehicle. Finally, while such electrical braking systems are sometimes supplied with an emergency breakaway kit, which includes a nonrechargeable dry cell battery which automatically completes the circuit to the electromagnet upon physical separation of the towed and towing vehicles, these emergency breakaway systems are completely dependent on the performance of such dry cell batteries which may, over a period of time, lose sufficient charge to properly activate the electric brakes.
A second type of conventional braking system used in conjunction with towed vehicles are surge hydraulic brakes. These types of systems conventionally include a piston and cylinder mounted between the towed vehicle and the towing vehicle in a manner such that when the brakes of the towing vehicle are applied, the momentum of the towed vehicle causes the piston to move into the cylinder resulting in hydraulic pressure which is transferred to the hydraulic brake cylinders of the towed vehicle. Disadvantages of this type of system include the apparent fact that in case of brake failure in the towing vehicle, no trailer brake pressure may be applied. Further, there is no provision for hand activation of such a system, thus making this type of system illegal under the relevant laws of some states. As is also apparent from the foregoing brief description of a conventional surge hydraulic braking system, full braking power is applied to the towed vehicle when the towing vehicle is operated in a reverse gear, whether such braking is desired or not.
The third type of conventional braking system is the vacuum over hydraulic brake system. This type of system conventionally includes a vacuum chamber, containing a diaphragm, mounted in the towed vehicle to which vacuum is supplied from the intake manifold of the engine of the towing vehicle. Upon activation of this type of braking system, atmospheric pressure is allowed to enter one side of the vacuum chamber, thus causing the diaphragm to expand into the evacuated side of the chamber, thereby mechanically moving a rod attached to the diaphragm which advances a piston in the master cylinder and creates hydraulic pressure to the hydraulic brakes of the towed vehicle. Release of the braking pressure is accomplished by reestablishment of a vacuum on both sides of the diaphragm within the vacuum chamber. The disadvantages of such a system include the necessity of leak proof vacuum lines to the vacuum source (usually the engine of the towing vehicle) and the fact that the operation of such systems is relatively slow causing a time difference between the application of the brakes of the towing vehicle and those of the towed vehicle. As is also apparent, complete loss of brakes in the towing vehicle will occur in the event of engine failure or separation or leak in the vacuum lines leading to the towed vehicle.